As a serious accident in a nuclear power plant develops, various different chemical processes lead to the generation of hydrogen. Because of this, combustible gas mixtures can form within the reactor containment. A release and build-up of hydrogen over a relatively long period can lead to explosive mixtures. This means an increased danger to the integrity of the reactor containment, the last barrier for retaining fission products. (Here the term "reactor containment" is used as the general term for all spaces in which the problem described can arise and must be solved.) Such gas mixtures can also appear in heavy-water moderated reactors. Furthermore, in the intermediate storage and final storage of spent fuel rods, hydrogen and its isotopes are also released into an oxygen-containing atmosphere and represent a certain hazard potential.
In order to avoid the danger stemming from such an explosive gas mixture, measures are known that are designed to remove the hydrogen in the gas mixture. These measures include the use of igniters as well as the catalytic recombination of hydrogen with the oxygen simultaneously present in the gas mixture so as to form water. (See, for example, published patent application EP-A-0 303 144). A particularly promising use seems to be that of catalytic recombiners (catalysts) which have become familiar in various configurations (see, for example, EP-A-0 416 143, DE-A-36 04 416, EP-A-0 303 144, DE-A-40 03 833). Published patent application DE-A-37 25 290 discloses the suitability of ternary palladium alloys such as PdNiCu as a catalyst for the above-mentioned purposes. Such catalysts can be used in the form of carrier bodies coated with the catalyst alloy, or else in the form of a spongy material or as granules.
The amount of hydrogen oxidized per unit of time by catalytic action increases exponentially with the temperature of the catalyst. The catalysts heat up due to the exothermic reaction until they reach an equilibrium between the heat generated and the heat dissipated. Only upon reaching relatively high catalyst temperatures does the removal of the hydrogen accelerate, and only then does the convection caused by the increase in temperature lead to an intermixing of the surrounding atmosphere.
FIGS. 1a and 1b show the temperature variation and the hydrogen-concentration variation, respectively, in a reaction chamber that contains such a catalyst. The measurement results shown were obtained under the following conditions. The catalyst consisted of a carrier plate of austenitic steel with a surface area of 0.8 m.sup.2 coated on both sides with a Pd alloy consisting of 95% by weight of Pd, 4% by weight of Ni, and 1% by weight of Cu. The spherical reaction chamber, with a volume of 10 m.sup.3, was first heated in a steam atmosphere. After reaching a temperature of 100.degree. C., the steam was pumped out of the reaction chamber and then 50% by volume of steam, 40% by volume of air, and 10% by volume of hydrogen were successively introduced.
As shown by the temperature variation pattern illustrated in FIG. 1a, the catalytic oxidation process commenced shortly after introduction of the hydrogen at time t0. Within about 7 minutes, the process caused the catalyst temperature to rise from 80.degree. to 560.degree. C. After reaching this maximum temperature of 560.degree. C. and sustaining an increase in hydrogen-concentration of up to 10% by volume (see FIG. 1b), both the temperature and the hydrogen concentration started to decrease because of the accelerated catalytic oxidation of the hydrogen at the higher temperature. The figures also show that, after reaching a temperature of about 160.degree. C. and a hydrogen concentration of about 2.8% by volume, no further decrease in the hydrogen concentration within the gas mixture could be perceived.
From the prior art are known so-called hydrogen-storage materials such as metals, metal alloys, or intermetallic compounds that can absorb hydrogen (and release it again) by means of a reversible process of hydride formation (see, for example, G. Sandrock, "Metal Hydride Technology Fundaments and Applications," Energietraeger Wasserstoff [Hydrogen Energy Sources], 1991 Annual Colloquium of the University of Stuttgart, VDI-Verlag Duesseldorf, 1991, pages 143-170). Until now, such hydrogen-storage materials have been used primarily for storing hydrogen as an energy source. From published patent application JP-A-63-072851, an alloy of zirconium with titanium, niobium, molybdenum, iron and vanadium is known that can function as a hydrogen-absorbing alloy. This material is used as a heat reservoir or temperature sensor as well as for the storage, transporting, separation, and purification of hydrogen.
The object of the present invention is to design an apparatus of the type indicated at the beginning such that a more complete removal of the free hydrogen, especially at relatively low temperatures, is achieved.